- #1

Stacyg

- 25

- 0

I know how to do simple coefficients using pascalles triangle but I really don't know how to do this.

Any help would be much appreciated.

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- Thread starter Stacyg
- Start date

- #1

Stacyg

- 25

- 0

I know how to do simple coefficients using pascalles triangle but I really don't know how to do this.

Any help would be much appreciated.

- #2

Tedjn

- 737

- 0

[tex](2x^2 - 3x^{-1})^3[/tex]

From Pascal's triangle, you know how to expand

[tex](a+b)^n[/tex]

What can you replace with a and what can you replace with b?

- #3

rock.freak667

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[tex](2x^2-\frac{3}{x})^3[/tex]

[tex] (\frac{1}{x}(2x^3-3))^3[/tex]

How about now?

[tex] (\frac{1}{x}(2x^3-3))^3[/tex]

How about now?

- #4

Tedjn

- 737

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Oh, that's a nice way of doing it :)

- #5

rock.freak667

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Oh, that's a nice way of doing it :)

Usually (well for me), a binomial expansion is usually done with a variable and a constant.

as for [itex](a+b)^n[/itex] is valid for [itex]|\frac{b}{a}|<1[/itex] But if a and b are variables, you'll have to do some fancy algebra to get the range for which it is valid.

- #6

Dick

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Usually (well for me), a binomial expansion is usually done with a variable and a constant.

as for [itex](a+b)^n[/itex] is valid for [itex]|\frac{b}{a}|<1[/itex] But if a and b are variables, you'll have to do some fancy algebra to get the range for which it is valid.

Why is it only valid in some range? I also don't see why you need to factor the original. (a+b)^3=a^3+3*a^2*b+3*a*b^2+b^3. Just put a=2x^2 and b=(-3/x), figure out which term is the x^3 term and evaluate it.

- #7

rock.freak667

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That's what I was taught.."validity of a binomial"

- #8

Dick

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That's what I was taught.."validity of a binomial"

Got a reference? If you are thinking of the convergence of the infinite series for negative exponents, that is something to think about. But this is a positive exponent, the series is finite. There are no convergence issues.

- #9

BrendanH

- 63

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Besides, we're dealing with polynomials in the case of (a+b)^n

- #10

Dick

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Besides, we're dealing with polynomials in the case of (a+b)^n

Yeah, that's what I mean by finite series. You could also just forget about pascal's triangle and multiply it out. The power is only 3.

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